Process of purifying cadmium



Patented Oct. 4, 1927.

HENRY HOWARD; or CLEVELAND, OHIO,

ASSIGNOR TO THE GRASSELLI CHEMICAL n COMPANY, OF CLEVELAND, OHIO, A CORPORATION OF OHIO.

- PROCESS OF PURIFYING CADMII IM.

17o Drawing.

This invention relates to a process of purifying cadmium and more particularly to a process of making pure cadmium or cadmium compounds from impure cadmium containing material such as cadmium sponge. The invention is directed particularly to the purification of cadmium sponge although, as will be apparent, it may be applied to any other analogous materials such as dross and turnings containing alkali soluble impurities such as zinc and aluminum.

Cadmium sponge is a metallic sponge which is usually made by dissolving the residues obtained as a by-product in the manufacture of zinc salts in sulfuric acid and treating the resulting solution with metallic zinc. The sponge is essentially metallic cadmium but usually'contains impurities such as zinc and iron.

In the preparation of pure cadmium or its compounds'fro'm cadmium sponge the impurities other than iron and zincare usually so small as to be negligible. The amount of iron present is also small and may be disregarded but if its removal is desired this presents no particular difiiculties. Zinc is the principal-impurity, being present in relatively large amounts, usually from 2 to 12 per cent depending upon the material used and the care employed in making the sponge.

My invention is therefore directed primarily to the separation of zinc from the cadmium content of the sponge and involves a simple commercial method for accomplishing this result.

My process consists in treating cadmium sponge or similar material containing cadmium and zinc with caustic alkali. preferably caustic soda solut on of about B. whereby the zinc content of the sponge and any other impurities soluble in caustic alkali are dissolved while the cadmium remains undissolved. In order to insure that the zinc is completely dissolved and that it remains in solution until the solution is separated from the sponge an excess of caustic alkali should be employed. Caustic alkali in quantity amounting to from two to five times the quantity theretically required to combine with the zinc content of the sponge in the form of zincate gives satisfactory results. The treatment of the sponge with caustic alkali is preferably carried out at elevated temperature in order to facilitate the dissolving of the zinc. For this purpose convenient material to store or ship I pifer Application filed April 24, 1922. SerialNo. 556,314.

the process is conveniently and satisfactorily carried out by boiling the mixture of caustic alkali solution and sponge. When the zinc content of the sponge has been brought into solution the liquid of the reaction mixture is diluted to about 20 B. and is separated from the undissolved residue by filtration, preferably on a vacuum filter and the residue is thoroughly washed with water. As is evident, alkali soluble mpurities other than zinc, if present, for instance aluminum. are also removed by the treatment described.

The residue is practically pure cadmium free from zinc. It may be used or sold in this form or if cadmium compounds are desired they may be obtained by well known methods.

As indicated above the crude cadmium sponge usually contains iron which is not removed by the described treatment with caustic alkali and is therefore present in the purified sponge or product of,the caustic alkali treatment. The removal of iron may be conveniently carried out during the conversion of the purified sponge to cadmium compounds. For this purpose the zinc-free sponge is dissolved in the minimum quantity of sulfuric acid and the solution is treated" with a small amount of a suitable oxidizing agent such as nitric acid to convert the iron content of the solution to the ferric state. The acidity of the solution is then reduced sutticiently to precipitate the iron content thereof preferably by dissolving therein cadmium oxid, hydroxid or carbonate} This precipitate is separated leaving a solution of'pure cadmium salts, principally the sulfate, free from metallic impurities. This solution may be regarded as a final product of the process but since the solution is not a to make from it a solid compound. admium carbonate and hydrate are convenient solid compounds of cadmium which may be readily produced in pure form from the iron and zinc-free cadmium sulfate solution by precipitation with alkali metal hydroxid or carbonate, the precipitate being then separated from the reaction mixture Washed and dried.

The cadmium carbonate or hydroxide product may be used as such or converted to other cadmium compounds or metallic no cadmium by known methods.

It is to be understood that. my invention is not limited to theentire rocess described orto the specific details of t e rocess except as is indicated in the appen' ed claims; and

while the claims are directed to the treat-.

ment of cadmium sponge, it is to be understood that the invention embraces the treat ment of other analogous materials.

I claim: 1. Process of purifying cadmium which comprises treating cadmium spqnge with caustic soda solution of 45 at its boilingt the 'zinc content of the cadmium sponge.

2.-Process of purifying cadmium. which comprises treat caustic alkali solution inquantityamounting to from 2 to 5 times the uantity theroretl e zinc content of temperature in quantity suflicient to dissolve v i cadmium sponge wlth amt HOWARD. I 

